Hybrid vehicles have recently been of great interest as environment-friendly vehicles. Some hybrid vehicles have come into practical use.
A hybrid vehicle includes, as its motive power sources, a DC (direct current) power supply, an inverter and a motor driven by the inverter, in addition to a conventional engine. Specifically, the engine is driven to generate motive power, and at the same time, a DC voltage from the DC power supply is converted to an AC (alternating current) voltage by the inverter to rotate the motor with the thus-converted AC voltage to thereby obtain motive power.
For the hybrid vehicle, it has been studied to boost a DC voltage from the DC power supply with a voltage step-up converter, and supply the boosted DC voltage to two inverters driving two motors, respectively.
In this regard, the hybrid vehicle is mounted with a motor drive apparatus as shown in FIG. 10. Referring to FIG. 10, the motor drive apparatus 300 includes a DC power supply B, system relays SR1, SR2, capacitors C1, C2, a bidirectional converter 310, a voltage sensor 320, and inverters 330, 340.
DC power supply B outputs a DC voltage. System relays SR1, SR2, when turned on by a control device (not shown), supply the DC voltage from DC power supply B to capacitor C1. Capacitor C 1 smoothes the DC voltage supplied from DC power supply B via system relays SR1, SR2, and supplies the smoothed DC voltage to bidirectional converter 310.
Bidirectional converter 310 includes a reactor 311, NPN transistors 312, 313, and diodes 314, 315. Reactor 311 has one end connected to a power supply line of DC power supply B, and another end connected to an intermediate point between NPN transistors 312 and 313, i.e., between an emitter of NPN transistor 312 and a collector of NPN transistor 313. NPN transistors 312, 313 are connected in series between a power supply line and a ground line. NPN transistor 312 has its collector connected to the power supply line, and NPN transistor 313 has its emitter connected to the ground line. Diodes 314, 315 are each arranged between the collector and the emitter of corresponding one of NPN transistors 312, 313 to cause the current to flow from the emitter side to the collector side.
Bidirectional converter 310, with NPN transistors 312, 313 turned on/off by a control device (not shown), boosts the DC voltage supplied from capacitor C 1, and supplies the output voltage to capacitor C2. In a regenerative braking mode of the hybrid vehicle mounted with motor drive apparatus 300, bidirectional converter 310 down-converts the DC voltage generated by AC motor M1 or M2 and converted by inverter 330 or 340, and supplies the resulting voltage to capacitor C1.
Capacitor C2 smoothes the DC voltage supplied from bidirectional converter 310, and supplies the smoothed DC voltage to inverters 330, 340. Voltage sensor 320 detects a terminal-to-terminal voltage of capacitor C2, i.e., an output voltage Vm of bidirectional converter 310.
Inverter 330, when supplied with a DC voltage from capacitor C2, converts the DC voltage to an AC voltage based on control from a control device (not shown), to drive AC motor M1. As such, AC motor M1 is driven to generate torque designated by a torque command value. Inverter 340, in receipt of a DC voltage from capacitor C2, converts the DC voltage to an AC voltage based on control from a control device (not shown), to drive AC motor M2. AC motor M2 is driven to generate torque designated by a torque command value.
Further, in the regenerative braking mode of the hybrid vehicle mounted with motor drive apparatus 300, inverter 330 converts an AC voltage generated by AC motor M1 to a DC voltage based on control from a control device, and supplies the converted DC voltage to bidirectional converter 310 via capacitor C2. In the regenerative braking mode of the hybrid vehicle, inverter 340 converts an AC voltage generated by AC motor M2 to a DC voltage based on control from a control device, and supplies the converted DC voltage to bidirectional converter 310 via capacitor C2.
Meanwhile, a system provided with a battery, a motor, and an electric power generator is disclosed in Japanese Patent Laying-Open No. 7-87614. This system is applied to a hybrid vehicle. In this system, the motor and the electric power generator are connected to the battery. The motor is driven by a battery voltage from the battery, and the electric power generator supplies generated electric power to an inverter driving the motor and to the battery. In an abnormal event of the battery where capacitance of the battery decreases and the motor cannot output a required torque, for example, the motor and the electric power generator are disconnected from the battery, and the motor is driven by the electric power generated by the electric power generator.
However, if the technique disclosed in Japanese Patent Laying-Open No. 7-87614 is applied when there is a fault in DC power supply B of motor drive apparatus 300, an overvoltage will be applied to the DC/DC converter connected between system relays SR1, SR2 and bidirectional converter 310, causing an unfavorable situation. In this case, increasing the withstand voltage of the DC/DC converter will lead to a cost increase. Further, when system relays SR1, SR2 are cut off while bidirectional converter 310 is conducting a switching operation, a ripple current may blow the contact points of system relays SR1, SR2.